In a typical Newtonain reflector-type telescope, a mirror, having a concave face, is located at one end of an elongated tube, or frame, and adjusted such that the concave face of the mirror will reflect and focus an image for viewing through an eyepiece axially disposed from the concave face of the mirror at an open end of the tube or frame of the telescope. By virtue of this arrangement, light entering the open end of the telescope from a distant object, such as a star or planet, is focused by the mirror and magnified by the eyepiece in such a manner that the distant object may be clearly viewed in a magnified state through the eyepiece.
It is well known that the image quality which can be achieved in a Newtonian reflector-type telescope is affected significantly by thermal boundary layers, created adjacent the concave surface of the mirror, when the mirror is at a temperature higher than the ambient air surrounding the mirror. Light from the distant object must pass through this boundary layer twice, once while approaching the mirror, and a second time after being reflected by the mirror, before reaching the focal point at the eyepiece. This phenomenon is discussed in significant detail, and a number of photographic and graphical illustrations are provided, in Attachment 1 to U. S. Provisional application, No. 60/733,027, the benefit of which is claimed herein, in an article from the September 2000 issue of Sky and Telescope magazine, entitled “Understanding Thermal Behavior in Newtonian Reflectors”, by author Bryan Greer.
In order to achieve an optimal image, with a given reflector telescope, therefore, it is necessary that the mirror be allowed to reach thermal equilibrium with ambient air surrounding the mirror. For a telescope which is normally stored inside of a building at an elevated temperature, it can typically take two to three hours or more for the mirror of the telescope to reach thermal equilibrium with the surrounding ambient air, after the telescope is moved outdoors for viewing stars and/or planets, for example. Having to wait this long in order to utilize the telescope is a source of considerable frustration to astronomers and star gazers. The problem of achieving thermal equilibrium is further exacerbated, in areas of the country where the ambient air drops significantly after sundown, such as in desert areas, where the nighttime temperature can easily drop ten to fifteen degrees in the hours after sunset. Where the mirror is of a large diameter, of six to eight inches or more, for example, the mirror may have too much mass to ever reach equilibrium with ambient air temperature, even in a non-desert area, during the nighttime hours.
The article by Greer provides a number of suggestions for designing a reflector-type telescope in a manner which will reduce the length of time required for cool-down of the mirror. Greer suggests utilizing a fan directed at the backside of the mirror, in a manner utilized in large Newtonian telescopes, for shortening cool-down times in smaller, portable, reflector-type telescopes.
The findings and suggestions of Greer are further developed in a mirror cooling apparatus disclosed in the January 2002 issue of Sky and Telescope magazine, in an article entitled, “Thermal Management In Newtonian Reflectors”, by Alan Adler, included in the form of Attachment 2 to the U.S. Provisional Patent Application 60/733,027 patent application. Adler discusses several approaches to utilizing a fan for enhancing cooling of the mirror of a reflector-type telescope, and recommends placement of one or more inlet fans in one side of the telescope tube in such a manner that air flow from the fan is directed at the front concave surface of the mirror, and exits out through the telescope tube through exit holes in the side of the telescope tube of a reflector-type telescope.
Greer also briefly address the problem of so called “tube currents,” which are caused when a mirror, which is warmer than the ambient air, sets up an air flow down along one portion of the inner wall of the tube and back up another portion of the inner wall of the tube. These tube currents cause distortion of the periphery of the image.
Furthermore, although not specifically addressed in the articles by Greer or Adler, under certain conditions, such as where a ground fog exists, where a telescope has been stored and/or transported in an air-conditioned building or vehicle, or where the ambient temperature rises during a viewing session to a point where humid ambient air is at a higher temperature than the mirror, it is possible to encounter conditions where the mirror may initially be at a temperature lower than the ambient outdoor temperature, and perhaps even be enough lower that condensation may tend to occur on the reflective face of the mirror. Under such conditions, it may be desirable to direct ambient air around and across the mirror in such a manner that the mirror is actually being heated by the ambient air, in a manner allowing use of the telescope soon after it is set up outdoors, and with the goal of maintaining thermal equilibrium between the mirror and the ambient air during a viewing session.
While the approaches disclosed by Greer and Adler, to utilization of a fan for expedited cooling and management of a thermal boundary layer adjacent the concave face of a mirror provide some improvement in addressing the problems described above, inter alia, these and other prior approaches are not entirely satisfactorily, and further improvement is desirable. For example, with the approach advocated by Adler, of mounting the fan to blow directly on the concave reflecting surface of the mirror, the fan is also blowing dust, moisture or other contaminates directly onto the reflecting surface of the mirror, thereby requiring that extreme care be taken to keep the mirror free of contamination, and that frequent, undesirable cleaning of the mirror be performed to maintain optimal performance of the telescope.
It is also noted, by both Greer and Adler, that the mirror mounting arrangement, commonly known as a cell, can also substantially affect the time required for the mirror and cell structure to reach thermal equilibrium with the ambient air. Directing air flow from a fan at or across the concave face of the mirror does little to help remove heat from the cell at an expedited rate.
It is desirable, therefore, to provide an improved method and apparatus for minimizing image distortion resulting from a thermal boundary layer adjacent the reflecting face of the mirror of a reflecting telescope, tube currents, and/or other impediments to clarity of image caused by lack of thermal equilibrium between the mirror of a reflecting telescope and the ambient air around the mirror, through judicious management of air flow across the reflecting face of the mirror, and through providing enhanced forced convection cooling and/or heating of the mirror and its related structures to bring the mirror into thermal equilibrium with the ambient air surrounding the mirror, at a rapid rate, and to maintain the mirror in thermal equilibrium with the ambient air while the telescope is in use.